Motor driven circular sanding devices are used in two different operational functions. One is a straight rotary action where the motor drive imparts a continuous, high speed rotation to a backup pad and the sanding or grinding disc attached to it. This form of operation is used in the heavy sanding or grinding of various materials such as metal, one example being the grinding of welds and metal from automobile bodies. Another form of operation is random orbital sanding where the motor drive, through an eccentrically located spindle and drive housing, imparts an eccentric or orbital path to the backup pad and sanding disc rather than a rotary action. In orbital sanding, there is no direct connection between the center stud of the backup pad and the housing of the drive head rotated by the motor means.
In conventional practice, an operator uses one machine as a high speed grinder with a rotary action to cut through heavy paint, welds and metal and then switches to another machine of a random orbital sanding type to smooth out the deep scratch marks from the previous grinding operation. In accordance with the present invention, both forms of sanding operations are available from one machine.
Double acting sanding heads are known, one example being shown in U.S. Pat. No. 3,482,362 of Dec. 9, 1969. However, the disclosure of this patent is, like several others of the prior art, directed to a device where the sanding operation must be stopped and the machine must be mechanically manipulated to change the form of action from one sanding function to another. In the present invention the switch from one form of action to the other only requires a decrease or an increase of pressure on the sanding apparatus by the operator.